This invention relates generally to a heat exchanger or a recuperator and more particularly to a heat conducting sheet used in making the heat exchanger or the recuperator.
Many gas turbine engines use a heat exchanger or recuperator to increase the operation efficiency of the engine by extracting heat from the exhaust gas and preheating the intake air. Typically, a recuperator for a gas turbine engine must be capable of operating at a temperature of between about 500 degrees C. and 800 degrees C. and internal pressures of between approximately 450 kPa and 1400 kPa under operating conditions involving repeated starting and stopping cycles. The exhaust gas normally determines the operating temperature and the intake air after being compressed normally determines the internal pressure.
Many recuperators are of a primary surface construction. In a primary surface recuperator, a plurality of sheets are stacked in a spaced apart configuration to form a cell. The spacing therebetween form a plurality of donor passages and a plurality of recipient passages. In many operations, the hot exhaust gas, between 500 degrees C. and 800 degrees C., is passed through the donor passages and an atmospheric temperature intake air is passed through the recipient passages. Although the atmospheric intake air may have passed through the compressor of the gas turbine engine, the temperature of the intake air is substantially below the 500 degrees C. to 800 degrees C. of the exhaust gas. Therefore, heat from the hot exhaust is transferred through the sheet and absorbed by the cooler intake air. Thus, thermal energy from the exhaust gas is extracted and conducted to the intake air increasing the efficiency of the engine.
In many applications the primary surface sheet used in forming the cell is very thin, flimsy and difficult to maintain a uniform cross sectional area of the passages between sheets. To enhance the rigidity of the thin sheets, the sheets are formed into an accordion type configuration forming peaks or crests and valleys. The peaks or crests and valleys form a plurality of upwardly and downwardly opening, transversely extending, relatively deep grooves being relatively closely spaced and having substantially vertical side walls or fins. In forming a recuperator using such sheets, the peeks of alternate sheets are aligned and the valleys of alternate sheets are aligned to form the donor passages and the recipient passages. Additionally, many of the sheets are formed with a serpentined configuration to enhance a controlled turbulent which increases heat conductivity and resulting efficiency. In manufacturing such recuperators, the component parts are fixedly attached together, usually by a welding process, to prevent leakage from the respective donor passages and recipient passages.
U.S. Pat. No. 5,060,721 issued on Oct. 29, 1991 to Charles T. Darragh discloses an example of one such recuperator. The recuperator disclosed in this patent has a circular configuration. The recuperator has a plurality of cell made from a pair of primary surface sheets, a plurality of spacer bars and a plurality of guide strips. The component parts are welded together to form the recuperator. The welding of these thin sheet and component parts into a cell having a sealed interface is difficult to accomplish in a cost effective and efficient manner.
During the operation of the gas turbine engine hot exhaust gas enters a portion of the recuperator, inlet of the donor passage, and cool atmospheric air enters another portion of the recuperator, the inlet of the recipient passage. The thermal stress placed on the components making up the cell and the recuperator causes the welds and components to fail after a number of cycles. To increase the number of cycles before failure, the materials, welds, assembly and assembly techniques need to be reviewed to overcome the thermal stress. For example, the hot donor fluid, exhaust gas, is at a temperatures of between about 500 degrees C. and 800 degrees C. and the recipient fluid, atmospheric intake air, is at or near an atmospheric temperatures of between about 0 degrees C. and 60 degrees C. Thus, the thermal difference or gradients experienced by the recuperator is extremely high. Thus, the thermal stress induced is also extremely high. Thus, a more effective and efficient use of materials and processes is needed to insure the increased longevity of the heat exchanger or recuperator.
The present invention is directed to overcome one or more of the problems as set forth above.
In one aspect of the invention, a primary surface sheet is adapted for use in a recuperator. The primary surface sheet has a first portion having a preestablished thermal deformation characteristic, the preestablished thermal deformation characteristic includes a resistance to high temperature deformation and a high temperature resistance to corrosion and a second sheet portion having a preestablished thermal deformation characteristic being less than the resistance to high temperature deformation and the high temperature resistance to corrosion than that of the first sheet portion are attached to form the primary surface sheet.
In another aspect of the invention, a cell is adapted for use with a recuperator. The cell has a plurality of primary surface sheets spaced apart a preestablished distance forming a fluid flow path. Each of the primary surface sheets have a first portion having a preestablished thermal deformation characteristic, the preestablished thermal deformation characteristic includes a resistance to high temperature deformation and a high temperature resistance to corrosion and a second sheet portion having a preestablished thermal deformation characteristic being less than the resistance to high temperature deformation and the high temperature resistance to corrosion than that of the first sheet portion. The first portion and the second portion are attached to form the primary surface sheet. A plurality of bars are interposed the plurality of primary surface sheets and the plurality of primary surface sheets and the plurality of bars are fixedly attached.
In another aspect of the invention, a method of making a recuperator is disclosed. The recuperator is made from a plurality of cell which are made from a plurality of component parts. The method of making the recuperator includes forming a primary surface sheet by attaching a first portion to a second portion, the first portion having a preestablished thermal deformation characteristic, the preestablished thermal deformation characteristic has a resistance to high temperature deformation and a high temperature resistance to corrosion and the second sheet portion having a preestablished thermal deformation characteristic being less than the resistance to high temperature deformation and the high temperature resistance to corrosion than that of the first sheet portion. The cell is formed by spacing a pair of the primary surface sheets apart a preestablished distance forming a fluid flow path and positioning a plurality of bars between the pair of the primary surface sheets. And, the pair of primary surface sheets are attached with the plurality of bars.
In another aspect of the invention, a method of making a primary surface sheet is adapted for use with a recuperator. The method of making the primary surface sheet includes attaching a first portion of the primary surface sheet to a second portion of the primary surface sheet. The first portion has a preestablished thermal deformation characteristic, the preestablished thermal deformation characteristic has a resistance to high temperature deformation and a high temperature resistance to corrosion and the second sheet portion has a preestablished thermal deformation characteristic being less than the resistance to high temperature deformation and the high temperature resistance to corrosion than that of the first sheet portion.